Piezoelectric crystal elements of shear mode

ABSTRACT

Preparations of piezoelectric single crystal elements involving the steps of mechanically finishing a single crystal element with select cuttings, coating electrodes on a pair of Z surfaces, poling the single crystal along the &lt;011&gt; axis under a 500V/mm electric field and a product made by the process thereof.

RELATED APPLICATIONS

This application claims priority from U.S. Ser. No. 12/252,037 filedOct. 15, 2008 issued as U.S. Pat. No. 7,908,722 on Mar. 22, 2011 whichis a continuation of and in turn claims priority from U.S. patentapplication Ser. No. 11/818,735 filed Jun. 15, 2007; which is acontinuation of and in turn claims priority from U.S. patent applicationSer. No. 11/182,704, filed Jul. 14, 2005; and which in turn claimspriority from U.S. Prov. App. Ser. No. 60/598,885 filed Jul. 14, 2004,the contents of each of which are full incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the piezoelectric crystal elements of shearmode and the process for the preparation thereof. The single crystalscan be PMN-PT (Lead Magnesium Niobate-Lead Titanate), PZMN-PT (dopedPMN-PT), or related piezoceramic materials. More particularly, thepresent invention relates to the discoveries of the new cut directionsthat optimize the shear mode piezoelectric properties. In the discoveredcut directions, the PMN-PT crystal elements and related compositionshave super-high piezoelectric performance with d₁₅, d₂₄ and d₃₆ sharemode at room temperature. Even more particularly, the present inventionrelates to a d₁₅ shear mode crystal that gives the maximum d value andis free from the cross-talk of d₁₁ and d₁₆. A further aspect of thepresent invention is that the d₃₆ mode provides substantial reliabilityover other shear elements due to its re-poling capability. The crystalelements above can be commercially used for high-sensitive acoustictransducers and in many other applications known to those of skill inthe piezoelectric ceramic and ceramic composition arts.

2. Description of the Related Art

The piezoelectric materials are the operational center of acoustictransducers which are broadly used in medical and commercial imagingsystems and SONAR systems. The most common types of transducers utilizelead zirconate titanate (PZT) based ceramics as a piezoelectricfunction. Piezoelectric ceramics convert mechanical energy intoelectrical energy and conversely electrical energy into mechanicalenergy. While conventional PZT materials remain the most commonmaterials used in acoustic transduction devices, changing materialrequirements have fostered the need for new piezoelectric materialshaving improved dielectric, piezoelectric and mechanical properties.

In THE early 1980s, Kuwata et al. (see J. Kumata, K. Uchino and S.Nomura, Dielectric and piezoelectric properties of 0.91Pb(Zn _(1/3) Nb_(2/3))O ₃-0.09PbTiO ₃, Jpn. J. Appl. Phys., 21, 1298-1302 (1982)) foundrelatively “high” piezoelectric coefficient, d₃₃, of 1500 pC/N andelectromechanical coupling factor, k₃₃, of 0.92 in 0.91PZN-0.09PT singlecrystals along <001> direction. The entire disclosure of Kuwata isincorporated herein by reference.

Later, relatively “high” piezoelectric properties were also observed inPMN-PT crystals by Shrout and his co-workers in 1990 (see T. R. Shrout,Z. P. Chang, N. Kim and S. Markgraf, Dielectric behavior of singlecrystals near the (1-x) Pb(Mg _(1/3) Nb _(2/3))O ₃-xPbTiO ₃ MorphotropicPhase Boundary, Ferroelectrics Lett., 12, 63-69 (1990)), but substantiallimitations remained during application and testing The entiredisclosure of Sprout et al., is herein incorporated hereby reference.

Reasonably “high” electromechanical coupling (k₃₃)>90%, piezoelectriccoefficient (d₃₃)>2500 pC/N and increased strain up to 1.7% in <001>orientation (poling along <001> axis) were reproducibly observed in thelater 1990's (see S. E. Park and T. R. Shrout, Ultrahigh strain andpiezoelectric behavior in relaxor based ferroelectric single crystals,J. Appl. Phys., 82, 1804-1811 (1997)). The improved “high” piezoelectricproperties noted in this literature promised a new application ofacoustic transduction devices using the longitudinal extension mode (d₃₃or compression mode) but failed to achieve the present results. Theentire disclosure of S. D. Park is herein incorporated by reference.

The shear mode of piezoelectric vibration is broadly used in acousticactuators and sensors. For examples, accelerometers utilizing the shearprinciple have some special advantages compared to the standardcompression type accelerometers as they are considerably less strainsensitive to mounting conditions. Unfortunately, the shear piezoelectriccoefficient d₁₅ for <001> oriented PMN-PT crystals is very small, lessthan 200 pC/N (see Rui Zhang et al., Elastic, piezoelectric anddielectric properties of multi-domain 0.67Pb(Mg1/3Nb2/3)O3-0.33PbTiO3single crystals, J. Appl. Phys. Vol. 90 (2001) 3471-3475). The entiredisclosure of Zhang is herein incorporated by reference.

However, the super-high shear piezoelectric coefficient d₁₅ for <111>oriented PMN-PT crystals was discovered as high as 8000 pC/N for PMN-33%PT crystal (Pengdi Han, Progress in PMN-PT crystal growth, 2002 U.S.Navy workshop on acoustic transduction materials and devices, 13˜15 May,2002 Penn State.) The entire disclosure of Han is herein incorporatedfully by reference. While this piezoelectric coefficient d₁₅ is oneorder higher than that of traditional PZT piezoelectric ceramics (themaximum d₁₅ of PZT-5H is typically 750 pC/N), this improvement limitedin understanding and nature, as will be discussed hereinbelow.

Soon after, it was confirmed that the d₁₅ could be as high as 4100 pC/Nfor PMN-30% PT crystals (see Rui Zhang et al., Single domain propertiesof 0.67Pb(Mg1/3Nb2/3)O3-0.33PbTiO3 single crystals under electric fieldbias, Appl. Phys. Letters Vol. 82 No. 5, February (2003)). The entirecontents of Zhang et al., are herein incorporated fully by reference. Aswith Han above, Zhang fails to provide a full understanding of theincreased d₁₅ measure.

Recently, the d₁₅ was also observed as high as 5980 pC/N for PMN-31% PTcrystals (see Jue Peng et al., Shear mode piezoelectric properties of0.69Pb(Mg1/3Nb2/3)O3-0.31PbTiO3 single crystals, Solid StateCommunications 130 (2004) 53-57). The entire contents of Peng et al. areherein incorporated by reference. Peng et al. fails to provide thenecessary understanding and additional elements to prevent cross talkand improve reliability.

US 2005/0034519 A1, Feb. 17, 2005 to Ken Kan Deng et al., the entirecontents of which are herein incorporated by reference) discloses anacoustic vector sensor, specially an underwater acoustic vector sensorusing a shear mode (d₁₅) PMN-PT crystal. However, as with each of thedisclosures noted above, there is no information of crystal orientationand cut direction details.

In view of the related references, it is clear to those of skill in theart that none provides a report of preparation and application for a d36shear mode of piezoelectric crystals.

As is also clear from the references themselves, all of the d₁₅'s testedor calculated above are based on the common orientation: <111> as polingdirection (3 axis) and <110> as applied field direction (1 axis). Thesereferences also illustrate the severe lack of investigation to determinean optimum direction (orientations) which give the optimizedpiezoelectric performance for each piezoelectric vibration modes.

As a consequence, there is a need to both optimize multiplepiezoelectric performance indicia and calculate an optimum direction. Inresponse to these needs, in this invention, we report the discoveryresults of the new cut directions that maximize piezoelectriccoefficients, including a d36 mode, for all of the possible symmetricdomain configurations of PMN-PT related crystals.

OBJECTS AND SUMMARY OF THE INVENTION

In response to the needs noted herein, it is therefore an object of theinvention to provide two kinds of shear mode piezoelectric crystalelements having the maximum of shear piezoelectric coefficient, i.e.,coordination rotated d15 and d36 shear mode, and preparation methodstherefore. They are:

-   -   A xzt−22.5° (±5° cut (<111> poling 3 m) d15 shear mode crystal        element free from the cross-talk from d16 and d11.    -   A zxt±45° (±5° cut (<011> poling mm2) d36 shear mode crystal        element having the re-polable characteristics:        -   A free X-Y cut (<111> poling 3 m) d15 shear mode crystal            element        -   A Y-cut d15 shear mode crystal element free from the            cross-talk from d16

According to the present invention, the piezoelectric crystal has thegeneral composition represented by the formula:PbZ_(y)(Mg_(1/3)Nb_(2/3))_(1-x-y)Ti_(x)O₃  (1)

where y is defined as 0 to 0.10, and x is defined as 0.20 to 0.35, and Zis represented by the one or more dopant elements. The dopant element(s)can be single elements or combinations of one or more of the elementslisted in Table 1.

TABLE 1 Dopants (used alone or in combination) Zr Hf Sn In Sc Tm Nb TaZn Yb Lu Sb Bi Mn Ga Ce Ni W Cu Fe K Na Li Ba

The present invention relates to Piezoelectric crystal elements havingpreferred cut directions that optimize the shear mode piezoelectricproperties. In the discovered cut directions, the crystal elements havesuper-high piezoelectric performance with d₁₅, d₂₄ and d₃₆ shear modesat room temperature. The d₁₅ shear mode crystal gives a maximum d valueand is free from the cross-talk of d₁₁ and d₁₆. The d₃₆ mode isextremely reliable compared to other shear elements due to its readyre-poling capability. The crystal elements may be beneficially used forhigh-sensitive acoustic transducers.

In application, the crystal elements above can be commercially used forultra-sensitive acoustic transducers and sensors, and in other marinerscommercial, military, and research orientated as known to those of skillin the art.

The above, and other features and advantages of the present inventionwill become apparent from the following description read in conductionwith the accompanying drawings, in which like reference numeralsdesignate the same elements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A provides for a transverse shear piezoelectric coefficient, d₁₅,a 3D plot of the piezoelectric surface of d₁₅. Here, Z=<1,1,1>,X=<1,−1,0> and Y=<1, 1, −2> and provides pseudo-cubic notation.

FIG. 1B shows a 2D plot and X-cut cross section of the piezoelectric d₁₅surface on the (110) plane indicating the occurrence of maximum d₁₅ andfree from d₁₆ cross talk.

The maximum d₁₅ obtained:

d₁₅=5192 pC/N at φ=0°, θ=−22.5°, and ψ=0°

d₁₅=−5192 pC/N at φ=0°, θ=157.5°, and ψ=0°

FIG. 2A shows a Z-cut plot of the piezoelectric surface of d₁₅.

FIG. 2B shows a Z-cut of a 2D plot of the piezoelectric surface of d₁₅indicating the independence of d₁₅ from cut direction rotating around Zaxis.

FIG. 2C shows the free XY-cut (<111> poling 3 m) for d₁₅ mode, angleTheta can be 0˜360°.

FIG. 3A shows a Y-cut of a 3D plot of the piezoelectric surface of d₁₅.

FIG. 3B shows a Y-cut of a 2D plot of the piezoelectric d₁₅ which showsa d₁₅ free of cross talk from d₁₆.

FIG. 4A shows 3D plot of the piezoelectric surface of d₃₆. Here,Z=<0,1,1>, X=<1,0,0> and Y=<0,1,−1> and provide a pseudo-cubic notation.

FIG. 4B shows A 2D plot, Z-cut cross section of the piezoelectric d₃₆surface on the (011) plane. The maximum d₃₆ obtained:

d₃₆=2600 pC/N at φ=45° or 225°, θ=0°, and ψ=0°.

d₃₆=−2600 pC/N at ψ=135° or 314°, θ=0°, and ψ=0°.

FIG. 5 shows the xzt−22.5° cut of d₁₅ mode for <111> poled PMN-PTcrystal.

FIG. 6 shows the zxt±45° cut of d₃₆ mode for <011> poled PMN-PT crystal.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The representation surfaces of the piezoelectric strain coefficient (d)were calculated for [011], [001] and [111] poled PMN-PT crystals with˜31% PT. It was discovered that the zxt±45° cut (rotation aroundz-axis±45° for [011] poled PMN-PT crystal gives a unique “re-poleable”shear piezoelectric coefficient d₃₆ up to 2600 pC/N.

The zxt 0° cut (without rotation) d₃₁ up to −1750 pC/N was obtained forthe [011] poled crystals. It was also found that an extraordinarily highshear piezoelectric coefficient d₁₅ up to 5190 pC/N for the singledomain crystal (3 m) occurred in the xzt−22.5° cut (22.5° clockwiserotation about x-axis). These calculated results were experimentallyverified, as will be discussed.

The transformation of piezoelectric coefficients by changing coordinatesystem is represented by the following equation:d′_(ijk)=Σa_(il)a_(jm)a_(kn)d_(1mn)  (2)

where d_(1mn) is the piezoelectric coefficient in the originalcoordinate system,

d′_(ijk) is the piezoelectric coefficient in the new rotated coordinatesystem,

and a_(il), a_(jm) and a_(kn) are the components of the transformationmatrix.

The coordinate rotation was defined in the following way: rotation wasfirst made by angle φ around the z-axis, then around the new x-axis byangle θ, and finally around the new z-axis by angle Ψ. All of therotations were counterclockwise. The new piezoelectric coefficientsafter the rotation in the 3-dimensional space were derived as functionsof the independent piezoelectric coefficients in the original coordinatesystem and the rotated Euler angles (φ, θ, Ψ) using tensor calculations.

To obtain the independent piezoelectric coefficients, three sets ofsamples of PMN-31% PT crystal (3 m, mm2, and 4 mm) were prepared to copewith the scattering of the measured data within each set caused by thePT-content variation and the process history.

The coordinates were selected as follows: [1 11] as z-axis, [110] asx-axis, and [11 2] as y-axis for 3 m symmetry; [011] as z-axis, [100] asx-axis, and [01 1] as y-axis for mm2 symmetry; and [001] as z-axis,[100] as x-axis, and [010] as y-axis for 4 mm, respectively.

An electrical field strength 5 kV/cm for poling was applied along thez-axis at room temperature. As used herein, room temperature rangesroughly from 33° F. to roughly 100° F.

The poling current density was limited within 10 μA/cm² by an automaticDC power supply unit. A complete poling can be achieved by retaining thepoling E-field for one minute after setting the poling current back tozero. The independent piezoelectric coefficients of the three engineeredmulti-domain systems were directly measured using a modified Berlincourtmeter with homemade adaptors. After repeated tries employing thissetting, it was determined that the present embodiment provides ameasurement error within about +5%.

A single domain PMN-PT crystal (3 m) can be obtained by completelypoling along the [111] direction. The single domain crystal has fourindependent piezoelectric coefficients: d₁₅ d₂₄), d₁₆ (=2d₂₁=−2d₂₂),d₃₁(d₃₂) and d₃₃. The representation surface of the shear piezoelectriccoefficient d₁₅ was then calculated, and is represented in FIG. 1. Asshown, the amplitude of the surfaces represents the absolute value ofthe piezoelectric coefficient in that orientation.

The maximum value of d₁₅ of 5190 pC/N is in the direction of θ of 337.5°and φ of 0° (xzt−22.5°). The maximum amplitude of d₁₅ (−5190 pC/N) wasfound at θ of 157.5° and φ of 0° (xzt 157.5°). The maximum d₁₅ value inthe rotated coordinate is approximately 1.1 times the original d₁₅.Particularly, the cross talk from d₁₆ is eliminated for the rotatedcoordinate. In contrast, strong cross talk between d₁₅ (4800 pC/N) andd₁₆ (1975 pC/N) exists before the rotation.

The shear piezoelectric coefficient d₃₆ is a dependent tensor and iszero in original coordinate circumstances. To explore the maximum valueof d₃₆ in a rotated coordinate system, the representation surface of theshear piezoelectric coefficient d₃₆ was calculated and this is shown asFIG. 4.

The maximum d₃₆ (±2600 pC/N) was obtained in the direction of θ of 0°and φ of ±45° (zxt±45°) or ±225°.

In an effort to verify the above maximum values from theoreticalcalculation, four groups of samples were prepared by cutting in therotation angle where the maximum d values had occurred. The measuredmaximum d values confirmed the calculation results, which are summarizedin Table 1 in context with the four types of vibration modes. Thecalculation on 4 mm multi-domains was not presented in this work, as ithas been initially described in a limited manner. described inreferences hereinabove and is incorporated here fully by reference.

It can be seen from the good consistency between the calculated resultsand the measured data in Table 1, that the present invention is easilyverified as valid.

TABLE 1 Provides a comparison between experimental value and calculateddata that validates the present invention. Vibration LongitudinalTransverse Longitudinal Transverse Mode Extension Extension Shear Shear

Symmetry 4 mm mm 2 mm 2 3 m Cut direction zxt 0° zxt 0° zxt ±45° xzt−22.5° Calculated d₃₃ d₃₁ d₃₆ d₁₅/d₁₆ value 2000 −1750 2600 5190/0(pC/N), 31% PT Measured d₃₃ d₃₁ d₃₆ d₁₅/d₁₆ value 2000 −1750 25205300/60 (pC/N), 31% PT

Referring now to FIG. 5, a process for preparation of the single crystalelement of the present invention comprises at least the steps:

-   -   (a) poling a single crystal with a selected composition, in the        direction along the <111> cubic axis under 500V/mm electrical        field at room temperature;    -   (b) mechanically finishing of the single crystal elements with        cuttings such as xzt−22.5°, ±5°; and    -   (c) coating working electrodes on both X surfaces and removing        the poling electrodes on both Z surfaces.

Referring now to FIG. 6, an alternative is provided for preparation ofthe single crystal elements described herein which comprises the steps:

-   -   (a) mechanically finishing of the single crystal elements with        cuttings such as zxt±45° (±5°);    -   (b) coating electrodes on a pair of Z surfaces; and    -   (c) poling the single crystal in the direction along the <011>        cubic axis under 500V/ram electrical field at room temperature.

A variety of experiments were conducted to test the aboveconsiderations. These experiments are discussed below.

Experiment 1

A plate crystal element, similar to that shown in FIG. 5, was createdand measured data of d₁₅ as high as 6,000 pC/N, and d₁₆ less than 100pC/N, and d₁₁ less than 90 pC/N.

Experiment 2

A plate crystal element, constructed as shown in FIG. 6, was measuredand provided measured data of d₃₆ as high as 2,000 pC/N and d₃₄d₃₅ lessthan 50 pC/N. The d₃₆ shear mode crystal elements was easily bere-poled, if any de-poling occurred or was necessary.

Experiment 3

The plate crystal element as FIG. 2, was provided wherein the rotationangle theta was taken from 0 to 330° in increments of 30°. The measureddata of d₁₅ are listed in Table 2.

TABLE 2 Experiment data for free X-Y cut (<111> poling 3 m) d₁₅ shearmode crystals Theta°    0°   30°   60°   90°   120°   150° d₁₅ pC/N 39403720 4050 3870 4100 4220 Theta°   180°   210°   240°   270°   300°  330° d₁₅ pC/N 4190 3788 4240 3870 4301 3904Experiment 4

In this experiment, a plate crystal element, as shown in FIGS. 3A and 3Bprovided measured data of d₁₅ as high as 4400 pC/N and d₁₆ less than 100pC/N.

Those of skill in the art should understand, that crystal cuttingorientation are described with PRE notation. Those of skill in thecrystal forming arts should additionally understand that the d_(ij)parameters were measured on a Berlincout type meter with an adapter anddielectric constant measured on a HP-4294A Impedance Analyzer.

Having described at least one of the preferred embodiments of thepresent invention with reference to the accompanying drawings, it is tobe understood that the invention is not limited to those preciseembodiments, and that various changes, modifications, and adaptationsmay be effected therein by one skilled in the art without departing fromthe scope or spirit of the invention as defined in the appended claims.

What is claimed is:
 1. A piezoelectric single crystal product made bythe process of: (a) providing a piezoelectric crystal element with acutting direction along zxt±45°; (i) said single crystal elementproviding a PbZ_(y)(Mg_(1/3)Nb_(2/3))_(1-x-y)Ti_(x)O₃; wherein Y isdefined as from 0 to at least 0.10, X is defined as 0.20 to at least0.35, and Z is defined as at least one doped element selected from thegroup consisting of Zr, Hf, Sn, In, Sc, Tm, Nb, Ta, Zn, Yb, Lu, Sb, Bi,Mn, Ga, Ce, Ni, W, Cu, Fe, K, Na, Li, and Ba; (b) said single crystalelement made by mechanically finishing said single crystal with cuttingsalong zxt±45°; (c) coating electrodes on a pair of Z surfaces; (d)poling said single crystal element to a first poled state in thedirection along <011> cubic axis under at least a 500V/mm electricalfield at room temperature and forming a poled single crystal element;and (e) providing said poled single crystal element with an operable d₃₆shear mode and having a d₃₆ value up to about 2000 pC/N at roomtemperature.
 2. A piezoelectric single crystal product made by theprocess, according to claim 1, further comprising the step of: (f)repoling said poled single crystal element in said first poled state toa second polled state.